Plants & Human Affairs (BIOL106)  -  Stephen G. Saupe, Ph.D.; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe

Seeds:  A Primer

 I.  Structure

• Embryo (radicle, hypocotyl, epicotyl, cotyledons)
• Seed coat (includes hilum)
• Food
       a.  provides nutrients for germinating seedling
       b.  kinds:  starch (cereals), fats & oils (nuts, soybean), protein (legumes)
       c.  amount varies:  lots (cereals, legumes) to little (orchids; rely on symbiotic relationship with fungi to support seedlings)
• Types
  • monocot (cereal)
  • "dicot" with endosperm
  • "dicot" without endosperm

II. Dormancy

A.  Definition

• dormant - suspended animation; won�t germinate even if conditions are favorable (innate dormancy)
• quiescence (enforced dormancy) � doesn�t germinate because conditions aren�t favorable
• primary � dormant immediately at harvest
• secondary (induced dormancy) � can germinate initially, but if exposed to adverse conditions (cold, low oxygen, high temp) will become dormant

B.  Are seeds alive?

• inactive metabolism, unable to detect
• viable vs. dead

C.  Function of dormancy

• withstand environmental extremes (e.g., cold, heat, radiation, microwaves)
  • loose water during development (dry to less than 10% moisture)
  • prevents ice formation
  • inactivates metabolism
• increase longevity - variable storage survival rates � few months to many years (10,000 arctic lupine); longest lived seeds with hard heavy coat or weeds; crops generally short viability
• provides time for dispersal
• space shuttle � SEEDS project, germinate after several years in space
• viviparous � a few with no dormancy, uncommon (i.e., mangrove), mutation in some

D.  Storage

• low humidity, dry
• air tight
• cool (i.e., seed bank storage, conserve germplasm, cryopreservation at -196 C)
• Millenium Seed Bank project - Kew Gardens

E.  Dormancy Mechanisms

1. Mechanical (heavy impervious seed coat) � scarification to break (file), acid, rotating drum with sandpaper (e.g., honey locust, morning glory,)
2. Chemical (inhibitors) � removed by washing out, chilling (e.g., ABA in ash other seeds, citric acid in tomatoes)
3. After-ripening (immature undeveloped embryos) � require growth period after being shed from plant (e.g., carrots, parsnips, hemp). 
4. Physiological inhibition
        (a) Light.  Stimulates many (esp. small seeds) or inhibits (uncommon) or no effect.  Acts via phytochrome, red light absorbing pigments, alternates between two forms)
         (b) Ethylene - larger seeds
         (c)  Cold - stratification, e.g., apples
         (d)  Heat treatments for desert and winter annuals (germinate after warm summer)
         (e)  Alternating temperatures - hot/cold; e.g., evening primrose, tobacco; mechanical change in seed coat or other mechanism; epicotyl dormancy - root emerges in warm temp, epicotyl requires cold, gives time for root to develop before epicotyl, e.g., wild ginger, waterleaf
5. Fire - increases light by reducing competition, destroy inhibitors in soil, charred remains stimulate, smoke stimulates - habitats that are seasonally dry and adapted to periodic burning (e.g., chaparral in CA, prairie in MN)

III.  Germination

A.  Requirements

1. water
2. oxygen
3. temperature
4. dormancy broken (chemical treatments to encourage seeds; e.g., GA, potassium nitrate)
5. suitable substrate (i.e., no inhibitors allelopathic agents present)
6. can you design an experiment to demonstrate these requirements?

B.  Events

1. absorb water (imbibition), swell (double volume, coat swells less, rips open, lots of force); initially quick, slows then followed by more rapid absorption; too fast damages cells, no time for 'repair'
2. activate physiological activities (food reserve breakdown, energy production via fermentation and ultimately respiration)
3. radicle (root) appears
4. beer making & hormones (malting, mashing, brewing, lagering)
5. can you design an experiment to measure the rate of imbibition?  or the impact of temperature on respiration? 

IV.  Planting seeds

1. Depth - no deeper than length or 3x the average diameter, shallower is better than deeper
2. Plant more than you think you need -  not all will germinate (can�t tell if dead, dormant or quiescent)
3. Thin as necessary - too much competition. Can you design an experiment to test the importance of thinning?
4. Methods -  Petri dish, pots, rag dolls, germination paper
5. Timing � important
   1. Cool Season (40 � 55 F or 4.4. � 13 C) � radish, lettuce, spinach, Swiss chard, beet, carrot, onion, cauliflower, cabbage, broccoli, kohlrabi, kale, turnips, rutabagas, peas, snapdragons, pansies 
   2. Warm season (> 60 F) -  tomato, egg plant, pepper, cucumber, squash, watermelon, cantalope snap bean, lima bean, sweet corn, marigold, zinnia
6. Seed bed prep - uniform eliminate clods to get good contact between seed and soil, free from weeds
7. When to plant indoors - transplants put out after the average last killing frost (in late May).  Tomato require about six weeks, annual flowers 6 � 8 weeks; cool season - sow outside as soon as work soil

V.  Seed Testing/Viability/Vigor

1. Germination Percentage = # seeds germ/total * 100
2. Germination Rate  %germ vs. time  (quicker better � sooner to photosynthesize, shorter growing time, uniform stand vs. uneven crop)
3. Viability Tests � sow, tetrazolium, float, cut open embryo
4. Gink & Go Study Seed Germination
5. Seed vigor - measure of seed quality, ability to germinate and grow

VI.  Seed production

1. strict controls
2. genetic uniformity and purity
3. fields typically isolated to minimize uncontrolled pollination/fertilization
4. harvest � when mature, ripe fruit collected prior to seed dispersal.  Various methods to tell � (color change in fruit � tomato or seed � apples black), changes in specific gravity � pine cones get lighter when ripe), density � acorns ripe sink
5. clean/extract if in fleshy fruit

VII.  Purchasing

1. Federal seed act � specifies standards for seeds entering commerce
2. pls � pure live seed
3. specify amount of weed seed
4. labels:  crop, cultivar or hybrid, if blended all listed
5. germination info, date
6. name/address of company
7. source (provenance) � for native plants, local sources (genotypes) best
8. Check out the Seed Packet questions

VIII.  Seed Types

1. seed tapes
2. pelleted seeds � encased in water soluble clay or talc, larger, easier to handle, can pre-treat, include fertilizers or other additives
3. primed seed - imbibed with water, early stages of germination activated
4. Terminator - engineered with gene that switches on to make seed sterile (prevents germination); therefore can't replant seed; protects patented varieties

IX.  Seedlings

1. first leaves
2. hook vs. sheath (dicot vs. monocot)
3. adult vs. juvenile
4. epigaeous vs. hypogaeous
5. initially heterotrophic - requires food from cotyledons until photosynthetically capable.  When do seeds become able to make food on own
6. small seeds - little stored reserves; i.e., orchids - rely on fungi

X.  Review

• click here for some review questions

Last updated:  11/04/2008 / � Copyright  by SG Saupe